Normal fullterm infants will be studied from 8 to 196 days of age while being breastfed or receiving various ready-to-feed experimental formulas of known composition. Bottles of formula will be weighed before delivery to the family and weighed again empty or partially empty to determine quantify consumed during each day of study. Anthropometric measurements and serum biochemical analyses will be performed at regular intervals. In general, formula-fed infants of the same sex will be assigned alternately to two feeding groups that will receive formulas that differ, in so far as possible, only in one respect -- e.g., energy concentration, type of carbohydrate, quantity of an amino acid, quantity of a trace mineral. In this manner it will be possible to identify various factors influencing food consumption by normal infants and to obtain additional information on nutrient requirements. A subgroup of infants being studied with respect to food intake and growth is also being studied for energy expenditures. Thh correlation between accumulated heart beats and oxygen consumption is determined in the laboratory and heart beats are then accumulated over 24 hours by use of a small, portable counter. From the number of heart beats and the established relation between oxygen consumption and heart beats, 24-hour oxygen consumption is estimated. Metabolic balance studies are carried out with normal infants to determine the interaction between minerals, including trace minerals, in influencing absorption and retention of these minerals.